Recent advances in recombinant DNA technology (including polymerase chain reaction protocols) and in somatic cell genetics, coupled with classic biochemical genetics and cytogenetics, offer an unprecedented opportunity to expand our knowledge of mammalian genome organization. The deer mouse (genus Peromyscus) is considered the ideal model for directed comparative mapping efforts for several reasons. For one, deer mice are extremely well characterized cytogenetically, and the mechanisms largely responsible for karyotypic evolution in primates and deer mice (pericentric inversions and heterochromatic additions) are apparently identical. Thus, any information obtained will have direct human relevance. With the exceptions of the laboratory mouse and rat, more clinical research is performed on the deer mouse than any other animal model. Expansion of the deer mouse gene map will have relevance and will facilitate new research initiatives in a wide range of biomedically important research areas. In addition, development of a deer mouse map will address the following fundamental biological questions.. 1) Does recombination inhibition within pericentric inversions function as a mechanism to protect gene combinations within these regions? 2) Is chromosome banding similarity among species indicative of underlying genic similarity; and 3) Have portions of the mammalian genome been conserved by chance or by selection?